The present invention is drawn to a process and catalyst for destruction of contaminants in a gaseous stream (preferably air) into less harmful products by irradiating the contaminated air with ultraviolet light in a first photocatalytic stage wherein the photocatalyst is irradiated with ultraviolet light while in contact with the contaminated air to convert at least a portion of the contaminants into carbon dioxide. The contaminated air then is passed into a second catalytic stage, wherein the contaminant is exposed to a catalyst without further heating of the contaminated air. Surprisingly, it is found that the pretreatment in the first photocatalytic stage increases overall process efficiency, by efficiently releasing heat utilized by the catalytic stage and by producing intermediate species which are efficiently converted in the second catalytic stage without forming harmful byproducts.
In spite of decades of effort, a significant need remains for an advanced technology to control stationary source emissions of volatile organic compounds (VOCs) as for example benzene, chlorinated volatile organic compounds (CVOCs) as for example trichloroethlyene, and toxic air pollutants (TAPs) as for example acrylonitrile. A particular need exists for technology which controls emissions from industrial processes and other applications where VOCs and TAPs are present in high flow rate air streams.
Pollution control in high flow rate air streams is becoming recognized as a major environmental control issue for the United States industrial community at large. For example, the control of emissions associated with solvent degreasing operations is necessary, including the emissions associated with exhaust ventilation fans. Also, air stripping of contaminated groundwater produces air emissions for which current technology provides no satisfactory solution. Catalytic combustors are available, but require processing tremendous volumes of air and result in uneconomic performance. Thermal incinerators require excessive supplemental fuel for dilute mixtures, and exhibit uncertain selectivity when CVOC's are involved. Gas membrane processes are only now emerging for gas separation, and are ill-suited for dilute mixtures. Pressure swing adsorption using zeolites or resins is not applicable to dilute mixtures, and rotating wheel adsorbers are uneconomic for such dilute concentrations of organics. Packed bed activated carbon adsorption is widely practiced, but creates a hazardous solid waste which is increasingly difficult to manage. Carbon regeneration by steam is costly, and is generally economic only for very large scale operations. Landfill options for spent carbon will become more limited, as it involves transportation and disposal of hazardous wastes, particularly for CVOC applications.
Control of indoor air pollution is also of growing importance, with the objective of enhancing workplace environmental health and safety protection. The Occupational Safety and Health Administration (OSHA) is promulgating new regulations to reduce workplace exposure to indoor air contaminants such as CVOCs. Many CVOCs are particularly toxic. Certain CVOCs are suspected carcinogens, others are linked to possible birth defects, and still others are suspected active precursors in the destruction of the stratospheric ozone layer. Of the 189 targeted air toxics in the Clean Air Act Amendments of 1990, about one-third of the compounds are chlorinated.
In spite of considerable efforts of researchers in the field, most UV photocatalysts exhibit shortcomings in catalyst activity, selectivity, and deactivation which limit their commercial utility for air pollution control.
Researchers have studied titania photocatalysis, although not achieving the benefits of the present invention. Raupp has reported titania photocatalysts for the UV oxidation of organics in air (see Raupp, G. B., et al., "Destruction of Organics in Gaseous Streams Over UV-Excited Titania", 85.sup.th Annual Meeting, Air & Waste Management Association, Kansas City, Jun. 21-26, 1992). The activity of a titania photocatalyst rapidly declines with time-on-stream with trichloroethylene (TCE) in air (see Raupp, G. B., "Photocatalytic Oxidation for Point-of-Use VOC Abatement in the Microelectronics Fabrication Industry", Air & Waste Management Association, 87.sup.th Annual Meeting, Cincinnati, Ohio, Jun. 19-24, 1995). Ollis describes photocatalytic reactors and reports that TCE photocatalysts in air can lead to 75 ppm(v) of phosgene in the reactor product in the photocatalytic oxidation of organics over a thin titania bed (see Ollis, D. F., in Photocatalytic Purification and Treatment of Water and Air, 481-494, Elsevier, N.Y., 1993; and Peral, J. and D. F. Ollis, "Heterogeneous Photocatalytic Oxidation of Gas-Phase Organics for Air Purification", J. Catalysis, 136, 554-564, 1992). Pichat has reported great difficulty oxidizing aromatics in the gas phase with a titania photocatalyst (see Pichat, P., in Photoelectrochemistry, Photocatalysis and Photoreactors, 425-455, Reidel Publishing, Boston, 1985). Researchers at Purdue have investigated gas phase photocatalysis of TCE using titania on a concentric reactor wall around a UV light source (see Wang, K. and B. J. Marinas, in Photocatalytic Purification and Treatment of Water and Air, 733-737, Elsevier, 1993). They employed residence times of over 6 seconds, and found evidence of byproducts, suspected to be phosgene. Teichner reports that byproduct formation with titania photocatalysts is the rule, not the exception (see Teichner, S. J. and N. Formenti, in Photoelectrochemisty, Photocatalysis and Photoreactors, 457-489, Reidel Pub, Boston, 1985). Nutech Energy Systems has disclosed titania impregnated on a fiberglass mesh (see U.S. Pat. No. 4,892,712 issued Jan. 9, 1990; U.S. Pat. No. 4,966,759 issued Oct. 30, 1990; and U.S. Pat. No. 5,032,241 issued Jul. 16, 1991). A technical paper on the Nutech technology disclosed a gas phase residence time of 8.4 seconds and evidence of byproducts formation up to 34 seconds residence time (see Al-Ekabi H., et al., in Photocatalytic Purification & Treatment of Water and Air, 719-725, Elsevier, N.Y., 1993). The University of Wisconsin investigators have used 100 second residence time for photocatalytic destruction of gas phase TCE using titania (see Yamazaki-Nishida, S., et al., "Gas Phase Photocatalytic Degradation on Titania Pellets of Volatile Chlorinated Organic Compounds from a Soil Vapor Extraction Well", J. Soil Contamination, September, 1994; Fu, X., W. A. Zeltner, and M. A. Anderson, "The Gas-Phase Photocatalytic Mineralization of Benzene on Porous Titania-Based Catalysts", Applied Catalysis B: Environmental, 6, 209-224, 1995; and U.S. Pat. No. 5,035,078 issued Jul. 30, 1991).
Titania has been used for decades in photocatalysis (see Formenti, M., et al., "Heterogeneous Photocatalysis for Oxidation of Paraffins", Chemical Technology 1, 680-686, 1971 and U.S. Pat. No. 3,781,194 issued Dec. 25, 1973). Due to the high absorption of UV light by titania, about 99% of the incident UV radiation is absorbed within the first 4.5 microns on titania (see Peral, J. and D. F. Ollis, "Heterogeneous Photocatalytic Oxidation of Gas-Phase Organics for Air Purification", J. Catalysis, 136, 554-564, 1992). Hence, the patent literature discloses methods to distribute the titania in thin layers in an attempt to overcome this deficiency. In 1973, Teichner disclosed a method of using titania supported on a matrix in a thin film reactor to oxidize hydrocarbons to aldehydes and ketones (see U.S. Pat. No. 3,781,194 issued Dec. 25, 1973). Titania has been deposited in thin layers on glass wool, on a ceramic membrane, and the wall of a reactor (see U.S. Pat. No. 4,888,101 issued Dec. 19, 1989; U.S. Pat. No. 5,035,078 issued Jul. 30, 1991; U.S. Pat. No. 4,966,665 issued Oct. 30, 1990). Raupp has disclosed titania for photocatalytic use when mixing two gas streams, and included thin bed catalytic reactors (see U.S. Pat. No. 5,045,288 issued Sep. 3, 1991).
The requirement of thin film reactors is a major deficiency, leading to greatly increased cost and complexity in commercial reactor construction. For example, a commercial reactor may require 500 to 1000, or more, UV lamps. Methods to provide illumination of thin films of catalyst require a huge surface area for impingement of incident UV radiation. For example, one common method of deployment of catalyst in a thin film is to coat the interior of a reactor tube, which surrounds a UV lamp in the centerline of the tube. This method, then, requires that 500 to 1000, or more, individual reactor tubes be constructed and assembled, which is costly and cumbersome. In addition, the flow must be equally split and balanced among the 500 to 1000, or more, reactor tubes in parallel operation, which is difficult and costly to accomplish and maintain. Further, to prevent the gas from bypassing the catalyst by preferentially flowing between the UV tube and the thin catalyst layer, the gap between the UV lamp and the catalyst must be very small or a turbulent promoting flow disruption must be inserted between the UV lamp and the catalyst. These lead to a very high pressure drop for the gas flowing through the reactor. Particularly for the large flow rates of gas in commercial reactors, this leads to high pressure drop, requiring both expensive gas compressors and a cooling device to remove the heat of compression of the gas. For these reasons, little or no commercial practice of thin film titania photocatalysts has been accomplished.
U.S. Pat. No. 4,966,665 discloses the use of titanium dioxide as a photocatalyst for destruction of chlorine-containing organic compounds in an oxygen-bearing vent gas. The method was used in a system wherein the titanium dioxide was supported on the wall of a reactor through which the vent gas is passed. This is an ineffective reactor design for large gas flows, requiring a very large number of small diameter parallel tubes among which the flow must be equally balanced. Further, the cost of such a reactor is very high compared to a single large diameter reactor. Still further, when a vent gas containing 30 ppm TCE was contacted with titanium dioxide for 26 seconds, 90% destruction of TCE was obtained. However, the analysis of the reactor products revealed the formation of byproducts such as 4 ppm phosgene and 1 ppm carbon tetrachloride which are more hazardous than TCE.
U.S. Pat. No. 5,045,288 discloses a method of removing organic contaminants from a gaseous stream, wherein a mixing step is conducted, combining (a) a gaseous oxygen bearing stream and (b) a contaminated stream. This mixture is then passed over a photocatalyst exposed to UV radiation of wavelength not greater than 600 nm. The reaction conditions must be preselected to prevent formation of a liquid phase on the catalyst. Only when the reactor comprises a body portion having a window for passing visible light, the photocatalyst may be selected from the group consisting of titanium dioxide, zirconium oxide, antimony oxide, zinc oxide, stannic oxide, cerium oxide, tungsten oxide, and ferric oxide. The invention is deficient in that it requires mixing two streams, requires avoiding liquid phases, and requires a visible light window, all of which are impractical in commercial systems. Further, the catalyst compositions are not effective.
U.S. Pat. No. 4,888,101 discloses a system for photocatalysis wherein a semiconductor is entrapped in a fiber mesh. U.S. Pat. Nos. 4,892,712 and 4,966,759 disclose a photocatalyst for detoxifying organic pollutants from a fluid, comprising a substrate in the form of a plurality of layers of a filamentous, fibrous, or stranded base material, and a photoreactive metal semiconductor material bonded to surfaces of said layers. The photoreactive material is selected from anatase, CdS, CdSe, ZnO.sub.2, WO.sub.3 and SnO.sub.2. This disclosure is of limited value because the substrate leads to a high pressure drop under conditions providing intimate contacting of the fluid flowing through or by the substrate. Further, the base material is generally of low surface area, leading to poor overall reactor performance. Still further, the design also generally requires a sleeve of this material around each bulb, leading to a costly reactor design. Finally, the photoreactive metal semiconductors are not of high photocatalytic activity. U.S. Pat. No. 5,032,241 provides a similar system for killing microorganisms in a fluid.
U.S. Pat. No. 4,780,287 discloses a method of decomposing volatile organic halogenated compounds by passing the gas through silica gel or quartz chips, and the bed is thereafter irradiated with UV light. Ozone or hydrogen peroxide are added to an aqueous phase. The system lost catalytic activity and required heating with nitrogen gas to restore the decomposing activity. Presumably, this loss of activity was due to the decomposition of the CVOC, rather than promoting its complete oxidation. U.S. Pat. No. 4,941,957 discloses a method for decomposing CVOC's present in gases and aqueous solutions. The aqueous phase CVOC's are removed by volatilizing into a gaseous carrier. The gas was then passed through silica gel and simultaneously irradiated with UV light and/or exposing the bed to ozone. The system required a great excess of ozone, using 2% ozone to decompose TCE at air compositions of only 50 parts per billion (ppb) to 700 ppb, which is generally uneconomic.
Accordingly, it is a principle object of the present invention to provide a process for efficient destruction of contaminants in a gaseous stream such as air utilizing a two step process.
It is a further object of the present invention to provide a process as mentioned above which comprises a photocatalytic step followed by a catalytic step.